The present invention generally relates to grounding structures which may experience adverse electrical effects, such as lightning. More specifically, the present invention relates to grounding outdoor support structures, such as light poles, by providing a low impedance path to ground.
It is well known that earth grounding is required for outdoor light poles as well as other structures per the United States National Electric Code (NEC), National Fire Prevention Association (NFPA), and most local codes. The general purpose of earth grounding such structures is to provide a path of low impedance such that electrical discharge from lightning or other sources may be dissipated to the earth with minimal damage to property or person.
Outdoor light poles as well as other structures are generally mounted to a concrete foundation, typically pre-cast or poured in situ, which interrupts the low impedance path to ground. For such structures NEC requires a copper or copper-clad earth grounding electrode of at least 8 feet in length to be buried to a minimum depth of 10 feet and connected to the light pole by a conductor sized appropriately per NEC code to complete the low impedance path to ground. If the measured resistance of the installed earth grounding electrode is greater than 25 ohms, then a second earth grounding electrode of at least 8 feet in length must be buried to a minimum depth of 10 feet and connected to the light pole by a conductor sized appropriately per NEC code.
Earth ground electrodes are generally provided and installed by the onsite contractor rather than the manufacturer of the outdoor structure or equipment to be installed on the structure. The contractor may not supply earth ground electrodes of the correct size and material, or may not drive the electrodes to the appropriate depth, or for a variety of other reasons, installation of the electrodes may be done incorrectly, or not at all. Improper installation of earth ground electrodes may lead to an insufficient impedance path to ground which may result in property damage.
It is also well known that various soil types demonstrate lower electrical impedance than others, particularly when moisture content is a factor. In certain soil conditions a resistance of 25 ohms can be difficult to achieve, even with appropriate installation of earth grounding electrodes per NEC code. Adding an additional earth ground electrode decreases the impedance path to ground but in cases of very poor soil conditions the overall earth grounding system may still exceed the 25 ohm resistance. Additionally, as has been stated, earth ground electrodes are typically provided by the onsite contractor and are not always installed correctly, so the consistency of the earth grounding system is limited from application to application.
A well known alternative to burying the earth ground electrodes in the soil is to bury the earth ground electrodes in the poured concrete foundation, known typically as an Ufer ground. For such structures NFPA and the Underwriters Laboratories, Inc. (UL) require a structural steel electrode of 20 feet to be buried in the concrete foundation and connected to the light pole or other structure by a conductor sized appropriately per NEC and NFPA code. Using the concrete foundation in this way increases the surface area in contact with the soil thereby decreasing the impedance of the earth ground connection. However, this alternate method of installing earth ground electrodes also relies upon the onsite contractor for consistency and correctness. Thus, there is room for improvement in the art.